Dry-type transformers for high voltage classes have been widely used in recent years in a number of utility and industrial installations because of their high reliability. Some of these dry-type transformers require the use of high voltages, high rated powers and a high regulating range, which lead to heating and hot-spot problems related to eddy and DC (or ohmic) losses in the windings of the transformer,
These eddy currents are induced by the magnetic flux generated by the current flowing through the winding, and they depend mainly on the module and direction of the magnetic flux: generally, it can be said that the more radial the magnetic flux, the higher the losses.
Also, in dry-type transformers requiring a high tapping range, when working in the lowest position of the transformer's tap-changer, high losses appear in the parts of the winding near to the connection points of the tap-changer, leading to a high hot-spot temperature within the zones surrounding said connection points.
In oil-type transformers, a regulation winding is employed to decrease hot spots created by the eddy currents along the winding; however, such a regulation winding may not be a suitable or appropriate solution for a dry-type transformer, since, because of its air-cooling system, it would require adding a very large and expensive regulation coil to the dry-type transformer.
The present invention aims to provide a dry-type transformer which solves at least partly the above drawbacks, by reducing the losses due to eddy currents, at least in the more problematic operating positions of the tap changer.